It will be the polymers causing the problem, generally too much water when cleaning up, more and more grouts are having this problem
Discuss Grout efflorescence in the Canada Tile Advice area at TilersForums.com.
Try using a colourant.
It should do if you clean before hand and use something like this Universeal Grout Rescue - Colour & Seal 237ml Black £16.50 in stock - next day UK delivery
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